/**********************************************************************
 *                   bootstrap 4 pull-right is invalid
 * Description.
 * When testing bootstrap, I found that pull-right is invalid, but it turns out that it has been replaced with float-right.
 *
 *********************************************************************/

I. Reference documents.
    1. What happened to the .pull-left and .pull-right classes in Bootstrap 4?
        https://stackoverflow.com/questions/39655110/what-happened-to-the-pull-left-and-pull-right-classes-in-bootstrap-4
    2. pull-right replacement in bootstrap 4 does not work
        https://stackoverflow.com/questions/46221635/pull-right-replacement-in-bootstrap-4-does-not-work

II. Solutions.
    The classes are currently float-right float-sm-right etc.